Field
Embodiments described generally relate to systems and methods for separating hydrocarbons using one or more dividing wall columns. More particularly, such embodiments relate to systems and methods for separating hydrocarbons using one or more dividing wall columns for use in a process for producing liquefied natural gas.
Description of the Related Art
Natural gas can be liquefied to facilitate its transportation. Prior to liquefaction, raw natural gas must generally be treated to remove components which can freeze and plug equipment during the formation and/or processing of liquefied natural gas (LNG). For example, water, carbon dioxide, and heavier hydrocarbon components containing five or more carbon atoms are generally removed.
It has typically also been desirable to fractionate natural gas into its various hydrocarbon components. Ethane, propane, and butane (C2-C4) are commonly used as refrigerants for natural gas liquefaction in the so-called multi-component or mixed refrigeration processes. Pentanes and heavier hydrocarbons generally have greater economic value as natural gas liquids (NGLs) or condensate for use in chemical feedstocks and gasoline. Fractionation processes typically involve cooling the natural gas to effect a partial condensation and feeding the partially condensed stream to a fractionation column commonly known as a scrub column. Methane can be taken primarily in the overhead vapor and heavier components can be removed primarily as a bottoms liquid. The bottoms are usually fractionated further into individual C2-C4 component products and a C5+ product. One or more of the C2-C4 products can be supplied as a makeup gas or liquid in the LNG refrigeration system (e.g., multi-component or mixed) and/or in order to make a liquefied petroleum gas (LPG) product. Typically, the scrub column employs an overhead reflux that, e.g., can be supplemented by a butane wash.
The mixed refrigerant (MR) process can be commonly employed in onshore LNG plants around the world. The MR process generally requires a continual supply of light hydrocarbons, e.g., methane, ethane, propane, isobutane, normal butane, isopentane, and normal pentane in order to function effectively and efficiently. These light hydrocarbons are typically distilled from the LNG plant feed gas in the fractionation section of the LNG train. The feed gas can be cooled as it passes through the LNG train and, once the feed gas reaches a low enough temperature, the light hydrocarbons condense from the gas stream. These hydrocarbons are then directed to the fractionation section, which can be a series of fractionation columns where each of the light hydrocarbons can be recovered as a single relatively pure component. These single components can be directed into the MR process, diverted to storage for future use, and/or directed back to the feed gas.
In a fractionation train of an LNG facility utilizing a mixed refrigerant (MR) process, for example, a propane mixed refrigerant (C3MR) process or a dual mixed refrigerant process (DMR), a typical C3MR or DMR fractionation unit can be generally required to produce ethane, propane, butane, and pentane refrigerants downstream of the scrub column. For example, the C3MR process generally includes a propane refrigerant cycle followed by a mixed refrigerant cycle where the mixed refrigerant includes methane, ethane, and propane. Also for example, a DMR process generally includes a first mixed refrigerant cycle where the mixed refrigerant includes methane, ethane, propane, butane, and pentane, followed by a second mixed refrigerant cycle where the mixed refrigerant includes methane, ethane, and propane. The fractionation column, e.g., a scrub column, fractionates a raw natural gas to provide a natural gas product and a hydrocarbon fluid having multiple components such as methane, ethane, propane, butane and components having five or more carbon atoms per molecule. Currently, the conventional C3MR and DMR fractionation units require three to six fractionation columns. For example, a conventional C3MR fractionation unit can include five columns including a scrub column, demethanizer, deethanizer, depropanizer, and debutanizer or, when utilizing a reboiler on the scrub column, can include four columns, i.e., the same five columns minus the demethanizer. Also for example, a conventional DMR fractionation unit can include six columns including a scrub column, demethanizer, deethanizer, depropanizer, debutanizer, and depentanizer or, when utilizing a reboiler on the scrub column, can include five columns, i.e., the same six columns minus the demethanizer. These multiple fractionation units can require significant capital costs to construct and operate, can require a large area of plot space, and due to the number of columns can require an increased number of personnel to operate that can also increase the potential for injuries.
There is a need, therefore, for improved systems and methods for separating a hydrocarbon fluid having multiple components to produce one or more hydrocarbons for use, e.g., during the production of a liquefied natural gas.